1. Field of the Invention
The present invention relates generally to electric vehicles. More particularly, the present invention relates to digital pulse width modulators with integrated test and control for electric vehicles. While the invention is subject to a wide range of applications, it is especially suited for use in electric vehicles that utilize batteries or a combination of batteries and other sources, e.g., a heat engine coupled to an alternator, as a source of power, and will be particularly described in that connection.
2. Discussion of Related Art
For an electric vehicle to be commercially viable, its cost and performance should be competitive with that of its gasoline-powered counterparts. Typically, the vehicle's propulsion system and battery are the main factors which contribute to the vehicle's cost and performance competitiveness.
Generally, to achieve commercial acceptance, an electric vehicle propulsion system should provide the following features: (1) vehicle performance equivalent to typical gasoline-powered propulsion systems; (2) smooth control of vehicle propulsion; (3) regenerative braking; (4) high efficiency; (5) low cost; (6) self-cooling; (7) electromagnetic interference (EMI) containment; (8) fault detection and self-protection; (9) self-test and diagnostics capability; (10) control and status interfaces with external systems; (11) safe operation and maintenance; (12) flexible battery charging capability; and (13) auxiliary 12 volt power from the main battery. In prior practice, however, electric vehicle propulsion system design consisted largely of matching a motor and controller with a set of vehicle performance goals, such that performance was often sacrificed to permit a practical motor and controller design. Further, little attention was given to the foregoing features that enhance commercial acceptance.
For example, a typical conventional electric vehicle propulsion system consisted of a DC motor, a chopper-type motor controller, an independent battery charger, and a distributed set of controls and status indicators. Vehicle performance was generally inadequate for highway driving, acceleration was uneven, and manual gear-changes were required. In addition, the issues of volume production cost, EMI, fault detection, maintenance, control and status interfaces, and safety were generally not addressed in a comprehensive manner.
There are two techniques for generating pulse width modulated (PWM) waveforms. The most common technique uses analog components while the other uses digital components. In an analog system, PWM waveforms are generated by an analog design using operational amplifiers and voltage comparators to compare crossing points of a triangular reference voltage signal with an applied control voltage waveform. The voltage comparators output the resulting PWM waveform. Then, an asynchronous dead-time circuit is used to produce a delay between the transitions of the PWM signal and the inverted PWM signal. Three sets of identical analog circuitry is required for a three phase system. However, the analog circuitry is prone to drift due to, for example, temperature variations, offsets, and gain variations that affect pulse width and cause channel-to-channel mismatches resulting in a reduction in the dynamic range of the motor control system. An implementation of a built-in test (BIT) circuitry needed to test the generation of pulse widths with nine bits of resolution is impractical. The use of digital circuitry would eliminate the disadvantages of the analog design such as drifts due to temperature variations and offsets.
Digital PWM waveform generation is available but this also has limitations. For example, in existing microcontrollers and digital processors that generate the PWM waveforms (1) a sawtooth reference waveform is used to produce the PWM signal rather than a triangular waveform, (2) a synchronous dead-time generation is not included with the processors, and/or (3) resolution is dependent on the PWM frequency and the resolution decreases as the frequency increases.
There are other stand-alone PWM integrated circuits that generate PWM waveforms, but these do not have synchronous dead-time generation, triangle based PWM generation, or individual control of the waveforms for built-in test. Also, these stand-alone PWM integrated circuits do not generate output pulses that are synchronized with the peak and/or valley of the triangular waveform.
In light of the foregoing, there is a need for an electric vehicle that has a pulse width modulator that overcomes the disadvantages of the related art.